I think the experience can vary quite a lot from production to production.
You usually meet friendly and interesting people amongst the other extras. Depending on where and when it is shooting you may have to prepare to be cold/wet/tired/bored. Extras are not always treated with particular consideration on a shoot and are rarely regarded as individuals by those "organising" you - you are just one of a "herd" in many cases. They may not bother to find out your name. It can be a very enjoyable way to earn a bit of money though.
Most of all I think you just have to be prepared for a heck of a lot of waiting around but you have to be ready to jump as soon as they want you. It's usually a very early start.
Take something to do or read.
Don't worry too much about not knowing what to do. You will be told what is needed of you - though I do think that the production team sometimes forget that the extras have not had the luxury of looking at the script, so occasionally the instructions can be difficult to interpret. If you have done filming work before then you will know all about on-set etiquette (keeping quiet, going back to starting positions on a new take etc). Professional extras tend to keep their heads down rather than hoping to be in shot - that way they can be used more often! It is considered bad form to talk to the actors unless they talk to you - or to the crew unless you need to (usually the 3rd AD is the person who will be talking to the extras on set) - and if it's a big film you will probably have very little to do with them (separate bus, separate time for make-up etc). You should not take your phone on to the set - nor should you take any photographs without express permission. You are also expected to wait until main cast and crew have been served their food. If it's a canteen rather than on location then you have to buy your own food usually, so make sure you take a bit of cash with you.
Check with the agency what you need to do about the paperwork - who you need to check in and out with (probably the 2nd AD) and what you need to get signed.
There's some pretty comprehensive advice on the Casting Collective's site which might be helpful even if you are not with this agency:
http://www.castingcollective.co.uk/arti ... hecking-in
I'm glad to hear that it's a great agency - but you may find you don't get all that much information from them other than call time and place. You should also be prepared (though I hope it won't happen to you) for it to be cancelled at short notice. Sometimes this is the evening before an early morning call.
I hope you have a great time - and please report back to NAPM and let us know how you got on!
